Field
The present disclosure relates to an organic light emitting display device, and more particularly, to an organic light emitting display device with improved operating voltage, light emission efficiency, external quantum efficiency, and lifetime characteristics.
Discussion of the Related Art
Image displays used for displaying a variety of information on the screen are one of the core technologies of the information and communication era. Such image displays have been developed to be thinner, lighter, and more portable, and furthermore to have high performance. With the development of the information society, various demands for display devices are on the rise. To meet these demands, research on panel displays such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescent displays (ELD), field emission displays (FED), organic light emitting diodes (OLED), etc is actively under way.
Among these types of flat panel displays, the OLED devices are advantageous in that they can be fabricated on a flexible substrate such as plastic, operate at a low voltage of 10 V or less, have lower power consumption, and deliver vivid color reproduction, as compared with plasma display panels or inorganic light emitting displays. Especially, white OLED devices are used for various purposes in lighting, thin light sources, backlights for liquid crystal displays, or full-color displays employing color filters.
In the development of white OLED devices, high efficiency, long lifetime, color purity, color stability against current and voltage variations, ease of manufacture, etc are important, so research and development are being done depending on which of these features should be taken into account. White OLED device structures may be roughly classified into a single-layer emitting structure and a multilayer emitting structure. Of these structures, the multilayer emitting structure having a blue fluorescent emitting layer and a yellow phosphorescent emitting layer stacked in tandem is mainly employed to realize white OLED devices with long lifetime.
Specifically, a stack structure of first and second light emitting parts is used, with the first light emitting part using a blue fluorescent diode as a light emitting layer, and the second light emitting part using a yellow phosphorescent diode as a light emitting layer. Such a white OLED device produces white light by mixing blue light emitted from the blue fluorescent diode and yellow light emitted from the yellow phosphorescent diode. A charge generation layer is formed between the first light emitting part and the second light emitting part to double the efficiency of current generated in the light emitting layers and facilitate charge distribution. The charge generation layer is a layer that generates a charge, i.e., electrons and holes, in it. The charge generation layer can prevent a rise in operating voltage because it doubles the efficiency of current generated in the light emitting layers and facilitates charge distribution.
However, the charge generation layer has a PN junction structure consisting of an N-type charge generation layer and a P-type charge generation layer. The difference in energy level between the P-type charge generation layer and the N-type charge generation layer deteriorates the property of injecting electrons generated at the interface between the P-type charge generation layer and an adjacent injection transport layer into the N-type charge generation layer. Also, if the N-type charge generation layer is doped with a dopant such as an alkali metal, the dopant is diffused into the P-type charge generation layer, thus leading to a decrease in the lifetime of the device.